How to Grow Watermelons from Seed (Citrullus lanatus)

Every summer someone tells me watermelons cannot be grown this far north, and every September I cut a Sugar Baby that proves otherwise. It is true that a watermelon (Citrullus lanatus) comes from the hot, dry summers of Africa: it wants hot days, warm nights and warm soil for three solid months, and a cool British or Dutch summer gives it none of those by default. But you do not need a southern climate — you need a greenhouse, a polytunnel or a south-facing wall, the right variety, and a sowing date that is later than instinct suggests. This guide covers the whole season: which varieties ripen in a short summer, when and how to sow, planting out, pollination, how many fruits to leave and the handful of mistakes that account for most failures. When the fruit is swelling, our companion guide When Is a Watermelon Ripe? takes over.

• Sow Mid-April to mid-May under cover · one seed per 9 cm pot · 25–30 °C to germinate
• Plant out Greenhouse or tunnel from mid-May · outdoors only in June, under a cloche
• Fruits per plant 1–2 large · 2–3 Sugar Baby-types · 4–6 minis
• Harvest Mid-August to late September · 70–90 days from sowing, depending on variety

Choosing a watermelon that ripens in a short summer

Variety matters more with watermelon than with almost any other crop, because the season is only just long enough. Small, early fruit ripens reliably; a 12 kg American giant usually does not. In our watermelon seed collection — also listed by species under Citrullus lanatus — the choices fall into three groups:

• Minis for pots and small greenhouses — Tigrimini F1 gives several striped fruits of one to two kilograms on a compact plant, with relatively few seeds, and is the most forgiving choice for a first attempt or a cool site.
• Round "icebox" melons — Sugar Baby, the classic dark-green, red-fleshed melon of around 3–4 kg, ripens in roughly 80 days and is the standard greenhouse watermelon for good reason. Janosik, an early variety with bright yellow, very sweet flesh, belongs in the same league.
• Large striped and long types — Crimson Sweet (also available as organic seed) and the elongated, grey-green Charleston Gray need 85–90 days and give the big picnic melons. Grow them in a tunnel or a large greenhouse, and only after you have succeeded with a smaller type.

Watermelons are close cousins of cucumbers, courgettes and pumpkins, and they are raised the same way, but they will not cross with any of them — nor with sweet melons, which are a different genus (Cucumis melo, in our melon collection). Growing them side by side in one greenhouse is perfectly fine.

When to sow watermelon seeds

The single commonest mistake is sowing too early. Some packets list February or March, and under a grow light in a heated room that can work, but a March seedling on a cool windowsill spends six weeks going nowhere, grows leggy, and is overtaken by an April sowing within a fortnight of planting. Watermelons dislike root disturbance and hate being held back in a small pot. Count back instead: sow three to four weeks before the plant can go into its final position. For a greenhouse or polytunnel that means mid-April to early May; for a sheltered bed outdoors under a cloche, the first half of May. You will find the right varieties in the seeds to sow in April collection, and our vegetable sowing calendar places them alongside everything else that starts that month.

Warmth is not negotiable. Watermelon seed germinates in five to ten days at 25–30 °C and barely at all below 20 °C. A heated propagator or a warm airing cupboard (checked twice a day, and moved into light the moment the hook of the stem appears) does the job; the principles are the same as in our guide to germinating seeds with water, warmth and light.

How to grow watermelon from seed, step by step

1. Sow one seed per pot Push a single seed 1.5 cm deep into a 9 cm pot of moist seed compost. Individual pots mean the root ball never has to be split, which watermelons resent more than any other cucurbit.
2. Keep it at 25–30 °C until it emerges Cover with a propagator lid or a clear bag. Water sparingly: cold, wet compost rots the seed before it sprouts. As soon as the seedling appears, give it the brightest spot you have, at 20–25 °C by day and no lower than 15 °C at night.
3. Harden off gently For plants going into a tunnel or outdoors, a week of gradual acclimatisation prevents the check that stops growth for a fortnight. Our guide to hardening off seedlings sets out the routine.
4. Plant into warm, rich soil When the plant has three or four true leaves, set it into a bed or large container enriched with well-rotted compost, the soil at least 18 °C and nights above 12 °C. Plant slightly proud on a low mound so the stem base stays dry. Allow about a metre per plant, or train minis and Sugar Baby up strings and nets in a greenhouse.
5. Pinch and train Pinch out the main tip after five or six leaves. Keep the three or four strongest sideshoots — these carry the female flowers — and remove the rest to keep air moving around the leaves.
6. Pollinate and thin the fruit Female flowers have a tiny striped melon behind the petals. If bees cannot reach them, pick an open male flower in the morning and brush it into the centre of the female. Once fruits reach tennis-ball size, keep only the number the plant can finish and remove any later flowers.
7. Water steadily, then ease off Water deeply at the base rather than over the leaves, and feed weekly with a high-potash tomato feed from the first swelling fruit. In the last fortnight before harvest, water a little less: the flesh concentrates its sugar and the rind is less likely to split.

Greenhouse or outdoors?

Outdoors, every degree counts. A black mulch sheet laid a week before planting warms the soil, and a cloche or fleece tunnel kept on until the plants flower buys most of the extra time a watermelon needs. Our guide to extending the growing season with cloches, fleece and cold frames shows how to set these up; lift the covers during flowering so that insects can reach the blooms. As the fruit swells, slide a tile or a pad of straw underneath to keep the rind off damp soil.

Common watermelon-growing mistakes

• Sowing in February or March without heat and light — the seedling stalls and is overtaken by an April sowing. Later is better.
• Sowing into cold compost — below 20 °C the seed sits, then rots. Use a propagator.
• Disturbing the roots — pricking out from a tray or pulling apart a pot of three seedlings costs weeks. One seed per pot.
• Leaving every fruit — a plant carrying six young melons ripens none of them properly. Thin hard.
• Choosing a giant for a cool site — start with Tigrimini or Sugar Baby, and graduate to Crimson Sweet once you have the conditions.
• Irregular watering — drought followed by a soaking splits the fruit and brings on powdery mildew. Water deeply and regularly at the base.
• Cutting on appearance — watermelons do not ripen after picking. Read the tendril and field spot as described in our ripeness guide.

Frequently asked questions

Can you grow watermelons in the UK or the Netherlands?
Yes, reliably under glass or polythene, and in a warm summer outdoors in a sheltered, sunny spot. The keys are an early-ripening variety, a sowing date in mid-April to early May, warm soil and limiting the number of fruits. A Sugar Baby or Tigrimini in a greenhouse will ripen in most years.

Can I grow a watermelon in a pot?
A mini variety can: use a container of at least 30–40 litres with rich compost, train the vine up a frame and support each fruit in a net or old pair of tights. Watering becomes the whole job in July, so a pot near the tap helps. Our guide to container gardening from seed covers compost and feeding.

Why are my watermelon flowers not producing fruit?
The first flowers on a watermelon are almost all male, and females — with the tiny melon behind the petals — follow a week or two later. If females appear and then shrivel, they were not pollinated: open the vents or hand-pollinate in the morning. A plant already carrying two or three fruit will also abort new ones, which is useful rather than a problem.

Can I save seed from my watermelon?
From an open-pollinated variety such as Sugar Baby, Crimson Sweet or Charleston Gray, yes — rinse the seed from a fully ripe fruit, dry it on a plate and store it cool. F1 hybrids such as Tigrimini will not come true. The general method is in our guide to seed saving, and the same principles of ripe fruit and slow drying apply to saving chilli seeds.

With this year's melons cut and eaten, the greenhouse is ready for its autumn clean — our end-of-season greenhouse checklist covers it — and the last chillies ripening beside it are worth a jar of seed: see how to save chilli and pepper seeds. Then, next April, sow a watermelon or two alongside something new from the melons, the fruiting vegetables or the full vegetable seed range, browse all our seeds, and keep our growing guides and the fruit sowing calendar to hand.